(1) Field of the Invention
The present invention relates to a semiconductor device and a production method thereof, and particularly to a MOS semiconductor device which enables reduction in gate leakage currents with a thinner gate dielectric film, and a production method thereof.
(2) Description of the Prior Art
As a material for gate insulator used in semiconductor devices which use MOS devices, silicon oxide has always been used when the gate dielectric film thickness (equivalent oxide thickness) as calculated based on oxide silicon permittivity is 3 nm or more.
However, as the effort to use a thinner gate dielectric film for higher speed has been accelerated, it has been found that, when its equivalent oxide thickness is 3 nm or less, direct-tunneling leakage current flowing through the dielectric film becomes considerable, resulting in an increase in gate leakage currents which is not negligible from the viewpoint of decreasing power consumption.
To solve this problem, there have been many attempts to reduce such direct tunneling leakage currents by increasing the physical thickness while maintaining the equivalent oxide thickness constant.
For instance, it has been reported on page 106 of IEEE Electron Device Letters Vol. 19 published in 1998 that a gate dielectric film which has a silicon nitride film formed on a thermally oxidized film formed on a silicon substrate or directly on a silicon substrate demonstrates reduced gate leakage currents when compared to oxide silicon film. However, the above-said report also discloses that it is necessary to perform annealing in an oxidizing atmosphere after formation of a silicon nitride film for reduction in leakage currents.
Usually, silicon nitride films are produced by chemical vapor deposition (CVD). It is known that in the silicon nitride film making process, the film contains much hydrogen and a bond of nitrogen and hydrogen serves as a charge trap. Therefore, it has been thought that although direct-tunneling leakage current can be reduced, the current through this charge trap is considerable, which thereby prevents leakage current reduction.
For this reason, it is a very important technique for gate leakage current reduction to decrease the charge trap in the silicon nitride by decreasing the amount of hydrogen in the film by annealing in an oxidizing atmosphere and, at the same time, adding oxygen.